This invention relates to fiber-reinforced composites, and more particularly to epoxy resin formulations having improved storage stability useful as matrix resins in the manufacture of fiber-reinforced composites.
Fiber-reinforced composites are high strength, high modulus materials which are finding wide acceptance for use in sporting goods and in producing consumer items such as appliances. Composites are also finding increased acceptability for use as structural components in automotive applications, as components of buildings and in aircraft. When used in structural applications, the composites are typically formed of continuous fiber filaments or woven cloth embedded in a thermosetting or thermoplastic matrix. Such composites may exhibit considerable strength and stiffness, and the potential for obtaining significant weight savings makes them highly attractive for use as a metal replacement.
The composites industry has long been involved in finding ways to further improve the mechanical properties of composite materials used in structural applications. Considerable effort has been expended over the past two decades directed toward the development of composites with improved fracture toughness. Inasmuch as most of the commonly employed matrix resins, as well as many of the reinforcing fibers, are generally brittle, much of that effort has gone into a search for components having better toughness characteristics. As a consequence, the search for toughened matrix resins has become the subject of numerous recent patents and publications, and numerous formulations have been made available to the composite industry through these efforts. Although the addition of rubber, thermoplastics and the like generally improves the ductility and impact resistance of neat resins, the effect on the resulting composites is not necessarily beneficial. In many instances the increase in composite toughness may be only marginal, and a reduction in high temperature properties and in resistance to environmental extremes such as exposure to water at elevated temperatures is frequently seen.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partly cured matrix resin. In order to be useful in commercial fabrication operations, prepreg needs to have a long out-time, defined as the period of time the prepreg can remain at room temperature and still be useful for making composites. For use in layups with complex contours the prepreg also must be pliable, and remain pliable in storage. Preferably the prepreg surface will also have and retain good tack. Pliability in prepreg is conferred by the matrix, which should remain soft and deformable to avoid cracking during fabrication.
The matrix resins most widely used for such prepreg systems are epoxy-based formulations, and many comprise an epoxy resin and aromatic amine hardener. The aromatic diamine hardener preferred for a wide variety of commercial applications has been 4,4'-diaminodiphenyl sulfone (DDS). DDS has a low level of reactivity with epoxy resins at room temperature, and prepreg made using DDS-based epoxy resin formulations generally has the desired long out-times. However, most epoxy matrix resin formulations based on DDS require further modification to overcome the low toughness that is characteristic of composites made from these resin formulations.
The isomeric form of DDS, 3,3'-diaminodiphenyl sulfone or 3,3'-DDS, is known in the art to be an effective hardener for epoxy resins. The reactivity of 3,3'-DDS is generally greater than DDS, and epoxy formulations based on this diamine generally have very short shelf life due to the greater reactivity. Although composites made from epoxy formulations based on 3,3'DDS are known to exhibit improved toughness, the shorter shelf life makes the manufacture of useful prepreg from such formulations a much more difficult task. Alternative diamines having lower reactivities, as well as a variety of cure inhibitors for use in slowing the cure rate of these highly reactive systems, have also become available to formulators of matrix resins, and some of these have found acceptance in the art. In order to produce fully-cured composites and attain the maximum possible toughness and resistance to environmental attack, many slow-cure systems require extended curing cycles and post-curing operations, and often require temperatures well above the 350.degree. F. curing temperature ordinarily preferred by the composite fabricating art. Such formulations are not preferred by fabricators, and have not been well-accepted.
The epoxy resin formulations presently available to the fabricator for producing toughened composites thus require further improvement. Formulations with extended shelf life and out-times would permit better handling and more practical storage for the resin and prepreg comprising the resin. If the formulations could be used in conventional fabricating operations with 350.degree. F. curing cycles to produce fully-cured toughened composites, they would represent a useful advance in the art and could find rapid acceptance by resin formulators and composite manufactures.